1. Field of the Invention
The present invention relates to a structure of a circuit board with an etching hole or slot in the power plane for eliminating the resonance effect and parasitic resonance on each trace.
2. Description of Related Art
A printed circuit board (PCB) is a circuit system with some specific functions and connected to a plurality of electronic elements like those in the telecommunications, high-frequency circuits, communication systems, aviation, ships, appliances, cars, machines and all kinds of instruments. For the technique advancement of IC, in which more conducting pins are required, the more complicated routing traces are also needed. As a result, the design of the circuit board is more complicated and concentrated, and a multi-layer circuit board with area reduction is in development.
The method for manufacturing the conventional PCB is to press a plurality of layers with different functions, including at least a power plane, an element plane, a signal plane, a ground plane and some interposed insulator layers. As the number of IC elements mounted on a PCB increases, these IC elements can be considered an element of equivalent resistance. The problem of voltage variation may occur when the current passes through the IC to induce a voltage drop. Therefore, a technique of plane routing is in development for promoting the efficiency in a PCB.
Nevertheless, since the distance of separation between the dielectric layer and the routing plane, power plane and ground plane is very narrow, a bad resonance effect and parasitic resonance is brought out. FIG. 1 shows the cross-sectional view of a multi-layer PCB in prior art, in which PCB 10 is formed with a multi-layer structure including a first routing plane 11 and a second routing plane 16 with the surface-mounted devices set thereon. There are even one or a plurality of inner routing planes 14 installed inside the PCB 10 if the area of surface routing plane is insufficient. Moreover, a first ground plane 12, a second ground plane 15 and a power plane 13 supply power to PCB 10.
In other words, the planes installed in the circuit board or the PCB can be treated as the parallel planes of a capacitor. The conducting planes and the decoupling paths therebetween can be equivalent to an inductance. Therefore the smaller the distance of separation between each plane is, the more violent the inductance-capacitance (LC) resonance will be. An example is given with the ground plane 12 and the power plane 13. The negative effect of a stray capacitor produced among these planes will severely eliminate the performance and frequency response of routing trace in a PCB.
Reference is made to FIG. 2, which shows a diagram for an example of the frequency response in a conventional PCB. It is apparent that there is a large-amplitude resonance occurring at 3.9 Ghz, as shown in the diagram, and this will decrease the quality of the circuit structure. In addition, for a PCB using a plurality of power sets, another parasitic resonance occurs at 2.9 GHz. Both conditions at 2.9 GHz and 3.9 GHz described above cause destructive resonance of a PCB in high frequency.
For improving the demerit of prior art, in which a severe resonance effect and parasitic resonance occur in current circuit boards at high frequency, so the present invention discloses a structure including one or a plurality power planes with specific etching holes to promote the quality of the circuit board.